The present invention relates to the art of curing catalysts for epoxy resins and particularly catalysts useful for powder formulations.
It is well-known to coat articles by applying a powder which contains an epoxy resin, a curing catalyst and optionally a curing agent to a substrate. Usually, either the substrate is heated and the powder is applied while it is still hot, or the powder is applied to the cold substrate and the substrate is heated afterwards. In either case, the heat causes the powder to melt and flow to coat the substrate and then to cure. Examples of suitable processes are described in Kaufman, U.S. Pat. No. 4,358,571 (Nov. 9, 1982) at Column 5, lines 5 to 49; Lee & Neville, Handbook of Epoxy Resins at Pages 20-15 to 20--20 (McGraw-Hill Book Co. 1967); and Tess, "Epoxy Resin Coatings," Epoxy Resins (2nd Ed.) at Pages 772 to 778 (Marcel Dekker Inc. 1988). For example, coatings are applied by electrostatic spray or fluidized bed to metal substrates, which are usually heated to 140.degree. C. to 240.degree. C.
Many curing catalysts are known for curing both epoxy powder coatings and solvent-borne coatings systems. Examples of suitable catalysts include tertiary amines and quaternary ammoniums, and tertiary phosphines and quaternary phosphoniums. Known latent catalysts contain a salt which contains an ammonium or phosphonium moiety and the conjugate based of a weak nucleophilic acid, such as boric acid or fluoboric acid. Examples of suitable catalysts and latent catalysts include lower-alkyl (C.sub.1 -C.sub.6)-triphenyl-phosphonium halide and the catalysts described in: Pham et al., U.S. Pat. No. 5,202,407 (Jan. 24, 1992); Bertram et al., U.S. Pat. No. 4,725,652 (Mar. 4, 1987); Bertram et al., EPO Patent Publication 0 328 020 A3 (Aug. 16, 1989); Muskopf et al., U.S. Pat. No. 5,140,079 (Aug. 18, 1992); Gan et al., U.S. Pat. No. 5,308,895 (May 3, 1994); and Bertram et al., U.S. Pat. No. 5,169,473 (Nov. 8, 1992). Kaufman et al., U.S. Pat. No. 4,358,571 (Nov. 9, 1982) teaches to make an adduct by reacting imidazole or a substituted imidazole with an acrylate ester, an epoxy resin or an isocyanate and then neutralizing the imidazole with a fatty acid or a dicarboxylic acid. These adducts are used as curing agents for epoxy resins at 132.degree. C. (270.degree. F.).
Burba et al., U.S. Pat. No. 5,175,219 (Dec. 29, 1992) teaches to: (1) react imidazolyl compounds with epoxy resin to make an adduct; and (2) to react the adduct with acrylic acid or its derivative in order to protonate amine hydrogen atoms in the adduct. The resulting adduct was reacted with epoxy resin to cure at about 120.degree. C.
Recently, it has become desirable to apply powder coatings to new substrates which cannot withstand high temperatures, such as wood or plastic. Common curing agents and catalysts are not useful for this purpose because they cure at too high a temperature. What are needed are curable epoxy formulations which do not substantially cure with the epoxy resin at ambient temperature and which melt, flow, consolidate and cure with epoxy resins to form good cured thermosets at temperatures which will not damage temperature sensitive substrates.
Additionally, it has become desirable to cure solvent-borne epoxy formulations at temperatures lower than the common curing temperature, in order to protect the substrate and to save the time and expense of heating formulations to very high temperatures. What is needed are curing catalysts and curable epoxy formulations which are stable at ambient temperature and which cure rapidly to form good cured thermoset resins at temperatures below ordinary epoxy curing temperatures.
One aspect of the present invention is a process to make a curable formulation containing the steps of:
(1) reacting an imidazole with an unsaturated compound containing at least one activated double bond to form a nucleophilic addition adduct; and PA1 (2) preparing a formulation which contains the nucleophilic addition adduct and an epoxy resin, PA1 (1) the nucleophilic addition adduct of an imidazole and an unsaturated compound containing at least one activated double bond, which adduct contains more than one imidazole moiety per molecule; and PA1 (2) an epoxy resin, PA1 (1) a nucleophilic addition adduct of an imidazole and an unsaturated compound; and PA1 (2) an epoxy resin, PA1 characterized in that: PA1 (1) the imidazole selected will react with the unsaturated compound by nucleophilic addition to form an adduct; and PA1 (2) the adduct will catalyze the curing reaction of an epoxy resin either by epoxy-epoxy curing or by reaction with a curing agent. The imidazole is preferably represented by Formula III: ##STR2## wherein each R.sup.2 is independently a hydrogen atom, an aliphatic moiety or an aromatic moiety, and each R.sup.3 is hydrogen or an aliphatic amine group such as a 3-aminopropyl group. Each R.sup.2 is preferably hydrogen or alkyl. Each R.sup.2 and R.sup.3 preferably contains no more than about 12 carbon atoms, more preferably no more than about 6 carbon atoms and most preferably no more than about 4 carbon atoms. Two R.sup.2 moieties on adjacent carbon atoms may optionally be linked to form a cyclic structure. Each R.sup.3 is most preferably hydrogen. Examples of suitable imidazoles include imidazole, 2-methylimidazole, 2-phenylimidazole, 2-ethyl-4-methylimidazole, and N-(3-aminopropyl)imidazole. PA1 (1) applied to a substrate by known means such as spraying, brushing, rolling, immersion or electrodeposition; and PA1 (2) cured by heating to a suitable curing temperature. PA1 (1) heating a substrate to a suitable curing temperature for the formulation; and PA1 (2) applying the formulation by known means such as electrostatic spray or fluidized bed. PA1 (1) applying the powder to the cold substrate, such as with an electrostatic application method; and PA1 (2) heating the powder and substrate to a temperature at which the powder flows and cures. PA1 (1) impregnating the formulation onto a fiberous substrate and heating to form a prepreg; and PA1 (2) pressing two or more layers of prepreg together at a temperature suitable to cure the formulation. PA1 (1) injecting the formulation into a mold, which optionally contains a fiberous substrate; and PA1 (2) heating the formulation to cure it.
characterized in that fewer than 50 equivalent percent of the imidazole moieties in the nucleophilic addition adduct are neutralized with acid prior to Step (2).
A second aspect of the present invention is a curable formulation which contains:
in a ratio of 0.02 to 10 equivalents of nucleophilic addition adduct per equivalent of epoxy resin, characterized in that fewer than 50 equivalent percent of the imidazole moieties in the adduct are neutralized with acid.
A third aspect of the present invention is the process of curing the formulation as previously described by heating it to a temperature at which the formulation cures, characterized in that the curing temperature is less than 130.degree. C. A fourth aspect of the present invention is a curable formulation which contains:
(a) the nucleophilic addition adduct is present in a catalytic quantity, and PA2 (b) the formulation further contains a curing agent for the epoxy resin.
Other aspects of the invention include the use of the nucleophilic addition adduct as a catalyst, use of the curable composition to make a coating, laminate or other composite or a molded article, and the article so made.
The adduct catalyzes epoxy-epoxy curing reactions and branching reactions. The formulations of the present invention can be cured at temperatures of about 130.degree. C. or higher to provide cured coatings which contain fewer bubbles than similar coatings which used common curing catalysts. In addition, formulations as described in the second aspect of the invention form stable powder-coating formulation which cure at temperatures below 130.degree. C. to provide low-temperature powder coatings for temperature-sensitive applications.
The present invention uses a nucleophilic addition adduct, which is made by reacting an imidazole with an unsaturated compound containing at least one double bond which is activated by an adjacent electron withdrawing group. For the purposes of this application, "nucleophilic addition" is used in the sense described in J. March, Advanced Organic Chemistry, 4th Ed. at Pages 741 to 743 (1992).
The unsaturated compound contains one or more activated double bond moieties (Q) per molecule. The activated double bond moieties (Q) are preferably linked to a common central moiety (A). The unsaturated compound is preferably represented by Formula I: EQU A--.paren open-st.Q).sub.n Formula I
wherein (A) is a central moiety as described hereinafter, each Q is an activated double bond moiety, and n is a number of unsaturated moieties bonded to the central moiety. The activated double bond moieties (Q) contain an aliphatic carbon-carbon double bond adjacent to an activating electron-withdrawing group. Examples of suitable electron-withdrawing groups include aldehyde, ketone, ester, amide, nitrile, nitrate and sulfonate moieties. Examples of preferred activated double bond moieties (Q) are shown in Formula II: ##STR1## wherein each R.sup.1 is independently preferably hydrogen, an aliphatic moiety, an aromatic moiety, or a link connecting the activated double bond moiety to an adjacent monomer. Each R.sup.1 is more preferably hydrogen or alkyl, and most preferably hydrogen or methyl. Each R.sup.1 is selected such that steric hindrance does not prevent the nucleophilic addition reaction. Each R.sup.1 preferably contains no more than 12 carbon atoms, more preferably no more than 6 carbon atoms and most preferably no more than 4 carbon atoms.
Each activated double bond moiety (Q) preferably contains an ester moiety, as illustrated in Formula II (a). It is more preferably an acrylate or methacrylate moiety.
The central moiety (A) may be a single unit or an oligomer or polymer which contains multiple repeating units. The selection of the central moiety is not critical as long as it does not interfere with the synthesis or use of the adduct. The central moiety preferably contains, for example, any one or more of the following: alkyl moieties, aryl rings, ether linkages, ester linkages, aliphatic or phenolic hydroxyl groups, glycidyl ether and/or ester moieties, acid moieties, or halogen atoms. It preferably does not contain moieties which cure with epoxy resins or catalyze their curing, such as: amine moieties, carboxylic acids, acid halides or acid anhydrides, thiol groups, or hydroxyl groups. The number average formula weight of the central moiety is selected to provide an adduct having a desirable softening temperature. The number average formula weight is preferably, for example, no more than about 5,000 and more preferably no more than about 3,000. It is preferably at least about 200.
The unsaturated compound preferably contains on average at least about 0.5 activated double bond moieties per molecule, more preferably at least about 1.0 activated double bond moieties per molecule and most preferably at least about 1.5 activated double bond moieties per molecule. The maximum number of activated double bond moieties per molecule is not critical, but in most cases is preferably no more than about 10, more preferably no more than about 6 and most preferably no more than about 4. Examples of preferred unsaturated compounds include: polyacrylates and polymethacrylates, unsaturated polyesters and vinyl ester resins. Other examples include alkyl, aryl and alkaryl acrylates.
The unsaturated compound is preferably a vinyl ester resin. The vinyl ester resin is preferably a reaction product of an advanced or unadvanced epoxy resin and an unsaturated acid. The epoxy resin is preferably a poly(glycidyl ether) and more preferably an advanced or unadvanced diglycidyl ether of a bisphenol. The unsaturated acid is preferably acrylic or methacrylic acid. The reaction preferably takes place in the presence of a catalyst, such as 2,4,6-tris(dimethylaminonethyl)phenol. Examples of suitable resins and processes to make them are described in Messick, U.S. Pat. No. 4,407,991 (Oct. 4, 1983) and Wykowski, EPO Publication 0 436 921 A1 (Jul. 17, 1991). The vinyl ester resin may optionally contain unreacted epoxy moieties. The equivalent ratio of vinyl ester moieties to epoxy moieties is preferably more than 1:1, more preferably at least 3:1, more highly preferably at least 10:1 and most preferably at least 20:1.
The unsaturated compound is reacted with an imidazole to form a nucleophilic addition adduct. The selection of imidazole is not critical as long as:
The ratio of imidazole to unsaturated compound is preferably selected to minimize the concentration of unreacted free imidazole in the adduct. The reaction mixture may contain a stoichiometric excess of imidazole, but it preferably contains no more than about 1 mole of imidazole compound per equivalent of activated double bond moiety and most preferably no more than about 0.95 moles. The minimum concentration of imidazole in the reaction mixture is governed by practical considerations, such as the desired concentration of imidazole in the finished adduct. The reaction mixture preferably contains at least about 0.5 moles of imidazole compound per equivalent of activated double bond moiety, and more preferably at least about 0.75 moles.
The temperature of the reaction is preferably at least about 50.degree. C. and more preferably at least about 100.degree. C. and most preferably at least about 120.degree. C. It is preferably no more than about 160.degree. C. and more preferably no more than about 150.degree. C.
The reaction preferably takes place in the presence of a polymerization inhibitor, such as hydroquinone or hydroquinone monomethylether, in order to prevent the unsaturated resin from gelling. Even so, some solutions are particularly sensitive to forming gels, so that special care must be taken. Sterically-hindered imidazoles, such as 2-ethyl-4-methylimidazole may react slowly and require additional stabilizer, such as hydroquinone, in order to provide sufficient time for the reaction to occur.
The resulting adduct contains .beta.-imidazole moieties, which are preferably represented by Formula IV (a) and more preferably represented by Formula IV (b): ##STR3## wherein each Z is an electron-withdrawing group as previously defined and each R.sup.1 and R.sup.2 has the definition and preferred embodiments previously given and each imidazole is preferably linked to a central moiety as previously described. The preferred number of .beta.-imidazole moieties in the adduct depends on the intended use of the adduct, and is similar to the preferred number of activated double bond moieties in the unsaturated ester compound, as previously described.
The adduct is preferably represented by Formula I wherein at least some Q are .beta.-imidazole moieties, and the remaining Q are activated double bond moieties. The equivalent ratio of .beta.-imidazole moieties to activated double bond moieties is preferably at least 1:1, more preferably at least 2:1 and most preferably at least 3:1. All of the activated double bond moieties may be converted to .beta.-imidazole moieties, but it is usually more practical that the equivalent ratio not exceed 20:1, in order to minimize free imidazole in the adduct. The adduct may optionally further contain reaction products of imidazole with epoxy moieties or other reactive groups in the unsaturated compound.
The softening point of the adduct is preferably high enough that the adduct is solid at ordinary storage temperatures, but low enough so that the adduct softens and consolidates with powder epoxy resin at the desired reaction temperature. The Mettler softening point of the adduct, as measured by the test set out in the Examples, is preferably at least about 50.degree. C., more preferably at least about 60.degree. C. and most preferably at least about 80.degree. C. It is preferably less than 130.degree. C. and more preferably less than 100.degree. C.
The number average molecular weight of the adduct is preferably at least about 400 and more preferably at least 500. It is preferably no more than about 1,500 and more preferably no more than about 1,100. The weight average molecular weight of the adduct is preferably at least about 400 and more preferably at least about 500. It is preferably no more than about 2,500 and more preferably no more than about 1,200.
The melt viscosity of the adduct (as measured at 150.degree. C. using an ICI Cone and Plate Viscometer with a type C cone) is preferably at least about 90 mPa.s and more preferably at least about 140 mPa.s. It is preferably no more than about 2,000 mPa.s and more preferably no more than about 1,500 mPa.s.
The adduct ideally contains essentially no (0 weight percent) unbound imidazole, which has not reacted with the unsaturated compound, but that result is frequently impractical to achieve. Preferably no more than about 50 weight percent of the imidazole is unbound imidazole, more preferably no more than about 30 weight percent and most preferably no more than about 20 weight percent. The minimum percentage of unbound imidazole is limited by practical considerations, such as steric hindrance, and is usually at least 1 weight percent.
In the prior art, imidazole moieties have typically been neutralized with an organic acid before they are made part of the epoxy formulation. In the present invention, at least a substantial portion of the imidazole moieties should not be protonated (neutralized). Preferably, at least about 50 mole percent of the imidazole moieties are unprotonated, more preferably at least about 75 mole percent are unprotonated, more highly preferably at least about 90 mole percent are unprotonated, and most preferably at least about 95 mole percent are unprotonated. As many as 100 percent may be unprotonated. Imidazole moieties which are unprotonated preferably exist in their free-base state.
Formulations of the present invention further contain an epoxy resin. The epoxy resin is preferably a glycidyl ether or ester compound, more preferably a glycidyl ether compound and most preferably an advanced or unadvanced diglycidyl ether of a bisphenol, such as bisphenol A or bisphenol F. The epoxy resin may be advanced or unadvanced, but it is preferably advanced and more preferably a solid at about 25.degree. C.
Its epoxy equivalent weight (EEW) is preferably at least about 100, more preferably at least about 200 and most preferably at least about 500. Its maximum EEW is not critical, but is preferably no more than about 2,500, more preferably no more than about 2,000 and most preferably no more than about 1,500. Its Mettler softening point is preferably at least about 50.degree. C., more preferably at least about 60.degree. C. and most preferably at least about 65.degree. C. Its Mettler softening point is preferably less than 130.degree. C. and more preferably less than about 100.degree. C.
Examples of suitable epoxy resins include epoxy powder coating resins, epoxy novolac resins, high- and medium-molecular weight solution epoxy resins, MDI-modified epoxy resins, glycidyl (meth)acrylate polymers or copolymers and liquid epoxy resins and blends thereof. Specific examples of epoxy resins useful in the present invention include bisphenol A, bisphenol F, and tetrabromobisphenol A. A broad range of suitable epoxy resins are commercially available. Processes to make others are familiar to persons skilled in the art and described in numerous general publications, such as Lee & Neville, Handbook of Epoxy Resins at Pages 2-1 to 3-24 (McGraw-Hill Book Co. 1967). The epoxy resins may also be the advanced products of liquid epoxy resins and tetrabromobisphenol A or the epoxy functional oxazolidone containing copolymers described in U.S. Pat. No. 5,112,932.
The optimum proportion of adduct to epoxy resin generally depends upon the contents and intended use of the formulation.
When the adduct catalyzes curing without a separate curing agent or cross-linker, the equivalent ratio is preferably at least about 0.02 equivalents of adduct per equivalent of epoxy resin, more preferably at least about 0.05 equivalents and most preferably at least about 0.2 equivalents. The maximum concentration is not critical, but is preferably no more than about 10 equivalents of adduct per equivalent of epoxy resin, more preferably no more than about 5 equivalents, more highly preferably no more than about 2 equivalents and most preferably no more than about 1 equivalent. The weight ratio of epoxy resin to adduct is preferably at least 1:10, more preferably at least 1:1 and most preferably at least 2:1. The weight ratio of epoxy resin to adduct is preferably no more than 10:1, more preferably no more than 5:1 and most preferably no more than 3:1.
When the formulation contains a curing agent and is cured at elevated temperatures, then a much smaller quantity of adduct is preferred. The equivalent ratio is preferably at least about 5 milliequivalents (meq) of adduct per equivalent of epoxy resin, more preferably at least about 20 meq and most preferably at least about 80 meq. The maximum concentration is not critical, but is preferably no more than about 200 meq of adduct per equivalent of epoxy resin, more preferably no more than about 150 meq, more highly preferably no more than about 100 meq and most preferably no more than about 50 meq. The weight ratio of epoxy resin to adduct is preferably at least about 1 parts per 100 parts resin (phr), more preferably at least about 3 phr and most preferably at least about 5 phr. The weight ratio of epoxy resin to adduct is preferably no more than about 15 phr, more preferably no more than about 10 phr and most preferably no more than about 5 phr.
Suitable curing agents for use in such formulations vary depending upon the intended use of the formulation, and are familiar to persons skilled in the art. Several suitable curing agents are taught in Lee & Neville, supra, at Page 20-11 and in Tess, supra, at Pages 776-778. Examples of suitable curing agents include dicyandiamide and other amines and amides, polyhydric phenols, and polyanhydrides. The optimum ratio of curing agent to epoxy resin varies depending upon the curing agent selected and the intended use of the resin. Usually, the equivalent ratio of curing agent to epoxy resin is preferably 0.1:1 to 10:1, and more preferably 0.2:1 to 2:1.
The formulation which contains sufficient adduct to catalyze epoxy-epoxy curing may further contain a curing agent. The equivalent ratio of adduct to curing agent is preferably at least 25:75, more preferably at least 50:50, more highly preferably at least 75:25 and most preferably at least 90:10.
The formulation may optionally contain a solvent, but preferably it does not and more preferably it is a powder coating formulation. The solvent, if any, is preferably organic. Suitable organic solvents are well-known and commercially available. The selection is not critical. Examples of suitable solvents include: xylenes, glycol ethers, ketones, toluene, alcohols and dimethylformamide. The concentration of solids in the solvent is not critical, but is governed by practical considerations such as viscosity, cost and the need to recover solvents from effluent. It is preferably between 20 and 80 weight percent and more preferably between 40 and 60 weight percent, in most cases.
The formulation may optionally contain other additives which are useful for its intended use. For example, coating formulations may optionally contain: stabilizers, surfactants and flow modifiers, fillers, pigments and matting agents. Laminate and composite making formulations may optionally contain stabilizers, fillers, flow-modifiers and chopped fibers. The concentration of additives other than pigments, fillers and chopped fibers in the formulation is preferably no more than about 5 weight percent and more preferably no more than about 3 weight percent. The concentration of chopped fibers, fillers and pigments is preferably no more than about 80 weight percent and more preferably no more than about 50 weight percent. The concentration of any or all may be 0 weight percent.
The formulation which cures by epoxy-epoxy homopolymerization preferably cures at a temperature of at least about 80.degree. C., more preferably at a temperature of at least about 90.degree. C. and most preferably at a temperature of at least about 100.degree. C. It preferably cures at a temperature of less than 130.degree. C., more preferably at a temperature of less than 120.degree. C. and most preferably at a temperature of no more than about 110.degree. C. However, it can be used at temperatures of 200.degree. C. or higher.
The formulation which contains a smaller quantity of adduct and a separate curing agent is preferably cured at a temperature of at least about 120.degree. C., more preferably at least about 130.degree. C. and most preferably at least about 150.degree. C. The maximum curing temperature varies depending upon intended use, but in most cases it is preferably no more than about 250.degree. C. and more preferably no more than about 220.degree. C.
Formulations of the present invention may be used in ordinary epoxy uses, such as coating, laminating and molding applications. For instance:
(a) Solution coating formulations may be: PA0 (b) Powder coating formulations may be applied by: PA0 (c) Laminates may be made by: PA0 (d) Molding may be made by:
They may also be applied by:
The catalysts of the present invention may also be used in electrical laminate application either via powder coating or solvent home systems such as described in U.S. Pat. No. 5,112,932.